The present invention relates to a testing device for a wind tunnel balance, and more particularly relates to a check calibration device for a sting-held strain-gauge wind tunnel balance.
Extensive amounts of wind tunnel testing time are devoted to measurement of the aerodynamic forces and moments acting on a test model. To measure these forces and moments, the test model is supported by a balance. Wind tunnel balances can be classified into two types: "external" balances which are located outside the model and test section, and "internal" balances which are located inside the model or its supports, or which may be integral with the model (or a portion of it) or the support. A generally used type of internal balance takes the form of a metal cylinder which fits inside of and supports the test model. This cylinder, which typically may be of 1/2 inch diameter by 3 inches long, is itself supported at the rear by a "sting" which can be made to change the attitude of the model. The balance itself is a complex assembly of strain-gauged elements arranged to sense directional and rotational forces acting on the model which it supports, the electronic signals from which elements may be fed to a computer to give a rapid readout. Careful calibration of the balance used to measure these forces is necessary to obtain the best possible accuracy, the objective being the determination of the constants in the equations chosen to represent the actual behavior of that particular balance. In a test environment, these equations are solved to provide the forces and moments corresponding to the recorded measurements. Balances may be damaged by aerodynamic overload or handling errors, and the initial calibration thereof should therefore be checked frequently.
The current method of checking the calibration of such a balance is to remove the balance from the tunnel and to mount it on an external calibration rig where incremental forces, generated by accurate weights, may be applied in the several modes, and the resultant electrical outputs measured. This is a tedious and time consuming process. In some larger tunnels it is customary to assembly a rig around the balance in situ. In addition to causing downtime of the tunnel, the handling of large weights in the tunnel can represent a problem. In either case, a full calibration of the balance may occupy two persons for two days.
There has been described another apparatus and method, designed for a large wind tunnel as a highly accurate standard for calibrating strain-gauge balances, comprising pneumatically powered force generators controlled by a computer, capable of generating any combination of loads in response to an input demand originating from a manually operated switchboard or from a stored programme. A large number of discrete load conditions are thereby imposed, requiring typically a calibration time of 12 hours. In that such apparatus must be restrained by the tunnel walls, allowance must be made for deflection of the sting.
It is an object of the present invention to provide an apparatus for testing the calibration of a device which will provide a less tedious and time consuming method of checking the calibration of a sting-held strain-gauge wind tunnel balance than the aforementioned method. It is a further object of the present invention to provide such an apparatus which will enable the checking of the calibration of such a balance in situ in the wind tunnel, such apparatus not being supported by the walls of the tunnel. It is yet a further object to provide a check calibration device for a balance which can indicate whether or not the calibration of the balance is accurate without having to completely recalibrate the balance.